Copper and copper base alloys are widely used in industry due to their good heat conductivity and corrosion resistance, such as pipelines of cooling systems and heat exchangers, but copper and its alloys usually suffer severe corrosion in acidic media. Using inhibitors to prevent copper corrosion is the most economical anti-corrosion method for many applications of copper. Benzotriazole (BTA, C.sub.6 H.sub.4 N.sub.3 H) and its derivatives (e.g., 1-methyl-benzotriazole, BTAM, tolyltriazole, also referred to as TTA or carboxybenzo-triazole CBT) are so far the best inhibitors in preventing the corrosion of copper and its alloys in industry. The patents pertinent to the BTA copper corrosion inhibition include U.S. Pat. Nos. 3,653,931; 3,791,803; 3,985,503; 4,744,950; 5,128,065; and 5,156,769. However, the inhibition efficiency of BTA and its derivatives drops dramatically in acid solutions, although the film formed in acid solutions is thicker than those formed in neutral and alkaline media. For example, the low inhibition efficiency of BTA/its derivatives in acid solution causes a problem in pickling (using acid solutions to dissolve the scales); too much copper is dissolved thus reducing service life. The low efficiency of commercially available inhibitors in acid also means that inhibitors are not available for preventing scale formation (e.g., calcium carbonate) in cooling systems which are nominally neutral but become acidic because of absorption of carbon dioxide and more corrosive toward copper. Similarly, Cu can be corroded by acidic rain, cooling, sea and potable water. In the case of slightly acidic potable waters, the copper levels in the water can become unacceptably high for consumption at levels well below those that cause degradation of the copper pipes. Therefore, efforts are underway to develop better inhibitors in these acidic environments.
Since the good inhibiting nature of BTA is known to be due to formation of a stable polymer film that occurs on copper surfaces in neutral and alkaline solution, efforts are underway to develop similar dense, thick films on copper in acidic media. BTA by itself is unsatisfactory in this regard.
Although potassium iodide (KI) has been used as an additive to improve the inhibition efficiency of some other organic inhibitors in preventing corrosion of irons and steels due to a synergistic effect, e.g., used with trans-cinnamaldehyde and alkynols to decrease the corrosion of steel in 20% HCl solution, it or other iodide compounds have not been adopted in copper corrosion prevention. Patents describing the corrosion inhibiting feature of potassium iodide, or iodide ion, include U.S. Pat. Nos. 2,559,580; 2,567,156; 3,816,322; 4,143,119; 4,640,713; and 4,851,149.
Although there were no suggestions that a combination of iodide compounds with BTA might increase the inhibiting property of the latter, we decided to explore such an approach and observed quite unexpectedly that such combination possessed superior anti-corrosion properties. This unexpected result led to further studies of the mechanism of the protective properties of the said combination of BTA and iodide ions. As mentioned earlier, the iodide ions are effective additives to some other organic inhibitors of corrosion of iron and steel. When used with trans-cinnamaldehyde and alkynols to decrease the corrosion of steel in 20% HCl solution, it is reported to do so by electrostatic attraction rather than film formation. Consequently, iodide compounds have not been adopted in copper corrosion prevention. The conventional explanation of the electrostatic attraction is that an attractive force exists between adsorbed anions on the metal and the organic cations, thus the adsorption of the organic cations on the metal surface is improved.
Our work has shown, however, that in the case of copper, the effects of the iodide ions on improving the inhibition efficiency of BTA/its derivatives is more complicated than simply an electrostatic force effect; the iodide ions actually form a polymer-like, dense, thick (400nm) film with copper and BTA/its derivatives, thereby possibly explaining the selective nature of BTA+KI (or other iodide compounds) for specifically functioning as a good inhibitor of copper, but not steels, etc. This multi-compound inhibitor provides a strong synergistic inhibition of copper corrosion in a wide variety of environments.
Most of the art that relates to the inhibition of corrosion of copper-bearing metals in aqueous systems requires the constant presence of the inhibitor in the aqueous medium. Only one of the examples cited, U.S. Pat. No. 4,744,950, addresses the method of inhibiting corrosion by formation of a stable and durable inhibiting film which does not require maintaining a level of inhibitor in the aqueous medium. The present invention provides an alternative to the protection of the copper bearing metals by formation of a protective film on its surface, which does not require maintenance of a level of inhibitor in the aqueous medium. The measured thickness of the formed film (400 nm) which is much greater than the reported thickness of the BTA film (5 nm) suggests a superior property of the composition and a method of using it as described in this invention.